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Electrocardiograph (ECG) is a test that is used to graphically represent the electrical activity of the heart. It is a simple and painless test. An electrical impulse or wave travels through the heart with each heartbeat.
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Keyterms: Electrocardiograph, ECG, Heart, Electrical impulse, Wave, Arms, Legs, Chest, Cardiac cycle
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What is Electrocardiograph?
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The term ‘electrocardiograph’ is made of three words, ‘electro’ – meaning electricity in Greek; ‘cardio’ – meaning heart in Greek and ‘graph’ – meaning “to write” in Greek. An electrocardiograph, also known as ECG, is a test that is used to graphically represent the electrical activity of the heart.
An ECG test helps to check the heart rhythm and therefore, detect any abnormality in the functioning of the heart. In a normal ECG test, the rate of heart beat is between 60 and 90 beats per minute (bpm). The PR interval is between 0.12 and 0.20 seconds; the QRS duration is between 0.06 and 0.10 seconds and the QT interval is less than 0.40 seconds.
(Electrocardiograph Machine)
Electrocardiograph Process
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- The ECG test is quite simple and painless. It checks the electrical changes – depolarization followed by repolarization – occurring in the heart of an individual during each cardiac cycle. For conducting an ECG test, the following steps need to be followed:
- Firstly, several small sticky electrical leads are attached to the patient’s arms, legs and chest.
- These electrodes are, in turn, connected to the ECG machine using wires.
- The electrodes record the smallest changes in the electrical activity of the heart during each cardiac cycle.
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Components of Electrocardiograph
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The main components of an ECG are as follows:
- P Wave: It represents the depolarization or electrical excitation of the atria. This, in turn, indicates atrial contraction. Normally, the P waves have a duration of less than 0.12 seconds and an amplitude of greater than 2.5 mm. An increased amplitude of the P wave indicates hypokalemia and right atrial enlargement whereas a decreased amplitude of the same might indicate hyperkalemia.
- QRS Complex: It indicates the depolarization of the ventricles, thus, resulting in ventricular contraction. The normal duration of the QRS complex is between 75 and 105 milliseconds. However, a prolonged duration of the QRS complex indicates hyperkalemia or bundle branch block. The QRS amplitude changes on the basis of the size of the ventricular chambers and the nearness of the chest electrodes to the same. Thus, it not only varies from one lead to another but also from person to person. An increased QRS amplitude is indicative of cardiac hypertrophy. The rate of heart beat of an individual can be determined by counting the number of QRS complexes in a given time period.
ECG Wave and QRS Complex
- T Wave: It indicates the repolarization of the ventricles, i.e., the return from the excited to the normal state. It also suggests the end of the systole. In a normal ECG test, the T wave is always upright and in the same direction as the QRS complex.
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Types of Electrocardiograph
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The different types of ECG tests are as follows:
- Resting ECG: It is a standard test conducted when the body of the individual is in the resting position. Thus, this test is used to determine the heartbeat of the patient while at rest. It is used for routine check ups to monitor the presence of any heart conditions.
Phases of Resting ECG
- Ambulatory or 24-hour ECG: It tests the heartbeat of an individual for a duration of 24 hours or even longer using a device known as Holter Monitor.
- Cardiac Stress or Exercise ECG: It tests the electrical activity of the heart of an individual during stress or exercise. It monitors the breathing and blood pressure (BP) of the individual. It also helps to determine the safe levels of exercise for patients suffering from various heart diseases.
Uses of Electrocardiograph
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An ECG test may be conducted for the following reasons:
- It can be easily used for diagnosing cardiac arrhythmias (abnormal rhythm of the heart)
- It can be used to detect any coronary artery diseases and take timely action for the same.
- It is used to monitor the health of the heart in case of patients suffering from diabetes, high blood pressure, high cholesterol, etc.
- It can also detect the previous occurrence of myocardial infarction.
- It can be used to monitor the side effects caused by any medication.
- It can be used to check the working of the mechanical devices implanted in the heart of the patient.
- It can be used to detect non-cardiac diseases like pulmonary embolism or hypothermia.
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Things to Remember
- An electrocardiograph is used to monitor the electrical activity of the heart of an individual.
- The P wave of an ECG indicates atrial contraction (depolarization).
- The QRS Complex of an ECG indicates ventricular contraction (depolarization).
- The T Wave of an ECG indicates ventricular repolarization. It marks the end of the systole.
- ECG is of three types– Resting ECG, Cardiac Stress ECG and Ambulatory ECG.
- An ECG test is used to detect cardiac arrhythmias and various cardiac and non-cardiac diseases.
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Sample Questions
Ques. Which component of the ECG indicates ventricular repolarization? (1 Mark)
Ans. The T wave of the ECG is used to indicate ventricular repolarization - the return from the excited to the normal state – of the heart.
Ques. What does the Cardiac Stress ECG test monitor? (1 Mark)
Ans. The Cardiac Stress or Exercise ECG is used to monitor the breathing and blood pressure (bp) of the patient. It also helps to determine safe levels of exercise for patients suffering from various heart diseases.
Ques. What is Resting ECG? (1 Mark)
Ans. Resting ECG can be defined as a standard test conducted when the body of the individual is in the resting position. Thus, this test is used to determine the heartbeat of the patient while at rest. It is used for routine check ups to monitor the presence of any heart conditions.
Ques. Mention four uses of an ECG test. (3 Marks)
Ans. The uses of an ECG test are as follows:
- It can be easily used for diagnosing cardiac arrhythmias (abnormal rhythm of the heart)
- It can be used to detect any coronary artery diseases and take timely action for the same.
- It can be used to monitor the side effects caused by any medication.
- It can be used to check the working of the mechanical devices implanted in the heart of the patient.
Ques. Which component of the ECG indicates ventricular repolarization? (4 Marks)
Ans. The T wave represents ventricular repolarization. Generally, the T wave exhibits a positive deflection. The reason for this is that the last cells to depolarize in the ventricles are the first to repolarize.
This occurs because the last cells to depolarize are located in the subepicardial region of the ventricles and these cells have shorter action potentials than found in the subendocardial regions of the ventricular wall. So, although the depolarization of the subepicardial cells occurs after the subendocardial cells, the subepicardial cells undergo phase 3 repolarization before the subendocardial cells.
Therefore, repolarization waves generally are oriented opposite of depolarization waves (green versus red arrows in figure), and repolarization waves moving away from a positive recording electrode produce a positive voltage.
Ques. Explain in detail the different components of an ECG. (4 Marks)
Ans. The main components of an ECG are as follows:
P Wave: It represents the depolarization or electrical excitation of the atria. This, in turn, indicates atrial contraction. Normally, the P waves have a duration of less than 0.12 seconds and an amplitude of greater than 2.5 mm. An increased amplitude of the P wave indicates hypokalemia and right atrial enlargement whereas a decreased amplitude of the same might indicate hyperkalemia.
QRS Complex: It indicates the depolarization of the ventricles, thus, resulting in ventricular contraction. The normal duration of the QRS complex is between 75 and 105 milliseconds. However, a prolonged duration of the QRS complex indicates hyperkalemia or bundle branch block. The QRS amplitude changes on the basis of the size of the ventricular chambers and the nearness of the chest electrodes to the same.
T Wave: It indicates the repolarization of the ventricles, i.e., the return from the excited to the normal state. It also suggests the end of the systole. In a normal ECG test, the T wave is always upright and in the same direction as the QRS complex.
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